I will be there next month at 28 weeks pregnant. But, I don't have any other children to care after at the same time - that might make me feel overwhelmed in wdw while 35 weeks pregnant in the heat. I have been to WDW many times at all months of the year, and grew up in this Florida Heat and humidity, so my body is accustomed to the FL weather. I will be there with my husband, brother and his family, and my father and his wife. So, they all know and understand the "rest" & hydration that I need. I just want to take a "Baby-Moon" and have fun with my family before the baby comes.
I think that your decision may depend on how you are feeling closer to the week of your trip. My pregnancy so far has been different with each passing week, between the nausea and bloating, although I have been able to keep up with going to the gym and exercising.
If you end up not feeling up to it, you can always postpone and go with all 3 kids. Maybe do something more local to you instead that could also be relaxing. Which ever you choose, congrats on your new lil blessing.